timekeeping: Make xtime and wall_to_monotonic static

This patch makes xtime and wall_to_monotonic static, as planned in
Documentation/feature-removal-schedule.txt. This will allow for
further cleanups to the timekeeping core.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
LKML-Reference: <1279068988-21864-10-git-send-email-johnstul@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>